Introduce ActionFunction to make it easy to patch and do extra validations before step starts (#365)

Co-authored-by: Shuchang Zheng <wintonzheng0325@gmail.com>

skyvern/exceptions.py

@@ -262,3 +262,8 @@ class BitwardenLogoutError(BitwardenBaseError):
 class UnknownElementTreeFormat(SkyvernException):
     def __init__(self, fmt: str) -> None:
         super().__init__(f"Unknown element tree format {fmt}")
+
+
+class StepTerminationError(SkyvernException):
+    def __init__(self, step_id: str, reason: str) -> None:
+        super().__init__(f"Step {step_id} cannot be executed and task is terminated. Reason: {reason}")